求翻译：Examples of tail-sitters are the United States Navy projects with Lockheed and Convair which utilize a turboprop power plant, and the USAF project with Ryan Aeronautical Corporation utilizing turbojet power plants. Examples of the flat-riser are the Rolls-Royce Flying Bedstead and the Bell VTO aircraft. The basic design problem associated with any aircraft of this type becomes one of achieving in a single vehicle VTO and military performance capabilities. A possible solution to this problem has been proposed by A. V. Roe, Canada, Limited, in the form of their Project Y2 (Secret).?是什么意思？

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